Li in NGC 6752 and the formation of globular clusters

¹ European Southern Observatory, Garching bei München, Germany e-mail: lpasquin@eso.org
² INAF – Osservatorio Astronomico di Trieste, Trieste, Italy
³ DASGAL – Observatoire de Paris-Meudon, France
⁴ INAF – Osservatorio Astronomico di Padova, Padova, Italy
⁵ INAF – Osservatorio Astronomico di Bologna, Bologna, Italy

Received: 9 June 2005
Accepted: 22 June 2005

Abstract

Li abundances for 9 Turn-off (TO) stars of the intermediate metallicity cluster ([Fe/H] = –1.4) NGC 6752 are presented. The cluster is known to show abundance anomalies and anticorrelations observed in both evolved and main sequence stars. We find that Li abundance anticorrelates with Na (and N) and correlates with O in these Turn-Off stars. For the first time we observe Pop II hot dwarfs systematically departing from the Spite plateau. The observed anticorrelations are in qualitative agreement with what is expected if the original gas were contaminated by Intermediate Mass AGB – processed material. However, a quantitative comparison shows that none of the existing models can reproduce all the observations at once. The very large amount of processed gas present in the cluster does not imply a “pollution”, but rather that the whole protocluster cloud was enriched by a previous generation of stars. We finally note that the different abundance patterns in NGC 6397 and NGC 6752 imply different composition of the pre-enrichment ejecta for the two clusters.